1. Field of the Invention
The present invention, entitled “Multimode Sample Introduction System,” is an apparatus for use in the field of scientific laboratory analysis. More particularly, the present invention relates to a combined spray chamber/gas-liquid separator for use in introducing samples into devices to measure elemental concentrations by atomic spectrometry.
2. Description of Related Technology
Atomic spectrometry is a technique that is applied to the determination of elemental concentrations whereby solutions of the species to be determined are delivered ultimately as atoms or ions in the gas phase where their concentrations are measured as a result of one of several physical processes. The principal methods of analytical atomic spectrometry include atomic absorption, atomic emission, atomic fluorescence and mass spectrometry. Commercial instruments are available for the determination of elemental concentrations by all four of the methods mentioned above.
Efficient delivery of the aqueous sample for determination of the elements has been a challenge for the atomic spectroscopy community and thus sample introduction has been described as the “Achilles' heel of atomic spectroscopy.” Analytical Chemistry, Vol. 56, pp. 787A-798A (1984). The principal reason for the problem arises from the need to deliver droplets of solution of a small diameter into the instrument. A consequence of the segregation of larger droplets is that a small portion only of the solutions (typically less than 5%) is delivered to the instrument. The remaining 95% is usually pumped to waste.
The concentrations of several elements in water, for example, are mandated by environmental regulation to be held at very low levels, which are difficult to measure accurately by most techniques. In metallurgical applications, the presence of some elements, such as germanium, bismuth and arsenic, alter important properties of metals and can improve or diminish advantageous characteristics of those metals and hence are regulated by industry standards.
Devices and methods for measuring and analyzing the concentration of elements and chemical compounds present in laboratory samples are well-known in the art. Among the preferred techniques for the determination of low levels of elements is the technique of vapor generation, in which a dissolved species, such as arsenic, in an ionic form, can be transformed into a species that is volatile. Such a form of the element can partition between the solution and the gas phase. In general, vapor generation involves the use of a gas-liquid separator for sample introduction purposes where certain analyte elements or compounds of interest are chemically converted into a vapor phase and the resulting vapor phase species are then stripped out of solution and delivered in the gaseous form. Although there are several volatile species that can be generated for this type of measurement from the vapor phase, the major species are hydrides, generated by the reaction of the ionic species in the aqueous solution. While vapor generation is limited in terms of the range of elements that are amenable to such a process, it provides significantly greater analyte transfer efficiency as compared to conventional nebulization (discussed further below).
Vapor generation has a long history. The first vapor generation test was developed by Marsh in the 1830s and was used for the determination of arsenic in cases of poisonings. Dedina, J. and D. L. Tsalev, Hydride Generation Atomic Absorption Spectrometry (1995). The sensitivity of the test persuaded researchers to use it to determine arsenic and later antimony in a variety of matrices. The vapor generation test for arsenic involved the reduction of arsenic to arsine in an acidic solution containing dissolving zinc. Researchers noted interferences from transition elements, and various techniques to minimize such interferences were reported during the early years following the development of the test.
The major advantage of the vapor generation technique was the separation of the analyte, in gaseous form, from the matrix. In the 1990s, the efficiency of removal of the analyte from solution was determined to be greater than 95 percent. Le, X. C., et al., 258 Anal. Chim. Acta, 307 (1992). From the first report, when Holak determined arsenic after cryotrapping, followed by flame atomic absorption spectrometry, improvements in detection limits were noted. Holak, W., 41 Anal. Chem. 1712 (1969). Since a limited number of analytes were transformed into volatile species, another benefit of vapor generation was realized when spectral interferences from line-rich elements (e.g., iron) were eliminated from the atom source.
Over a period of years, mercury, germanium, tin, selenium and tellurium were added to the list of analytes determined to be amenable to vapor generation. Dedina, supra. More recently, in particular over the last two decades, the list of elements that can be determined by being transformed into vapor phase species has grown considerably. Lead, cadmium and thallium were determined from their hydrides, nickel was determined by transforming it into its tetracarbonyl, and osmium was determined from its volatile oxide. Within the last five years, several more elements, including Ag, Au, Co, Cr, Cu, Fe, Mn, Ni, Pd and Rh, were added to the list of elements that can be determined from volatile species. While it is not clear in what form some of these elements are delivered to the excitation source, it is clear that mass transfer efficiencies are significantly greater than those from solution nebulization.
All of the foregoing advantages aside, vapor generation has often proved difficult and problematic. Problems identified by various researchers include the following: poor reproducibility of results (i.e., high relative standard deviations (RSDs)); need for separate introduction systems for vapor generation and nebulization; limited number of analytes amenable to such processing; complex chemistry; transfer-line problems (e.g., condensation, catalytic decomposition of species); difficulty of understanding mass transfer processes from the gas-liquid separator; and complicated nature of the chemistry of vapor generation and interferences. For example, gas-liquid separators commonly encounter the problem of elevated RSDs, due to the nebulization of solution during the vapor generation reaction, which causes the formation and bursting of bubbles of hydrogen (or of carrier gas, in the case of frit-based and similar systems). Such effervescence entrains droplets into the gas stream, which, in turn, can give rise to uneven and unpredictable spikes in concentration of volatile species.
Numerous inventions over the last 35 years have sought to improve the delivery of elements into the vapor phase. Many devices produce noisy signals in the instrument, thereby reducing the efficiency of the measurement and making it more difficult to measure very low concentrations. Most devices (usually called gas-liquid separators) depend upon the generation of a gas, usually hydrogen, in the solution, which strips the volatile species from solution. Such devices use a reagent (usually a solution of sodium borohydride NaBH4), which mixes with an acidified solution of the sample and generates both the vapor phase species and hydrogen simultaneously.
A variety of gas-liquid separators has evolved over the years. Holak's approach was to trap the generated hydride in a U-tube cooled with liquid nitrogen and subsequently desorb it. Holak, supra. In addition, the Thompson U-tube and various frit-based separators have been described over the years. Dedina, supra. One recurring issue is the “dead volume” of the gas-liquid separator. Perkin-Elmer developed two devices that are useful for reducing such dead volume. Brindle and Zheng compared several designs for gas-liquid separators for the determination of mercury, including a model detuned nebulizer (i.e., one with poor nebulizing properties). Brindle, I. D. and S. Zheng, “A Comparison of Gas-Liquid Separators for the of Mercury,” 51 Spectrochimica Acta, Part B, pp. 1777-80 (1996). CETAC Technologies, Inc. developed a gas-liquid separator that uses a glass post onto which the premixed reaction mixture is pumped. A tangential flow of argon is used to strip the volatile species as the liquid flows along the post.
Another methodology for determining the level or concentration of one or more chemical compounds or elements in a laboratory sample is nebulization, which typically involves use of a cyclonic spray chamber to atomize or aerosolize the target solution into tiny droplets that become briefly suspended in said chamber. In short, nebulization is a process whereby a solution is transformed into an aerosol. This nebulization process is most frequently achieved by passing high velocity gas past or over a capillary that carries a solution. The liquid is propelled into the gas phase as droplets of various sizes. The diameter of the droplets is a function of the design of the nebulizer and the flows of gas and solution into it. A second device, usually called a spray-chamber, is used in atomic spectrometry to segregate the finer particles (usually particles of a size less than approximately 10 micrometers) from the larger particles, which are allowed to coalesce and be drained away. The small droplets are carried by the gas flow to the atomic spectrometry instrument.
It has been demonstrated that the introduction of a nebulized solution of potassium chloride simultaneously with vapor generated species results in a significant increase in signal of volatilized species from the sample. Brindle, I. D. and X-C. Le, 61 Anal. Chem. 1175 (1989). In such circumstances, the potassium served to enhance the signal from the analytes by the so-called easily ionized element effect.
In the late 1990s, technicians at Jobin-Yvon, Inc. (JY) attempted to develop a device that would allow the generation of vapor phase elements and determine them concurrently with conventional nebulization of analytes in a cyclonic spray chamber. See http://icpoes.com/cma.htm. Reduced species are generated in a reservoir (created through the use of an elevated drain) located in the base of a modified gas-liquid separator, where excess hydrogen (caused by the use of a high concentration of acid, together with the reagent, sodium tetrahydroborate (III), also called sodium borohydride) sweeps out the vapors into the gas stream to be carried off to the excitation source. Using a novel flow system, and incorporating a focused microwave cavity for heating solutions, the JY technicians were able to report the determination of As, Bi, Ge, Hg, Pb, Sb, Se and Te. Id. While such methodology resulted in reported improvements in detection limits over conventional nebulization, the approach was marked by significant drawbacks, including the fact that the JY device: (1) requires high acid concentrations to be effective, (2) requires a specific protocol for the determination of elements, and (3) is designed specifically for JY optical spectrometers, whereas the present invention has broad applications to optical and mass spectrometers for elemental determinations.
A Japanese patent from 1989 (no. 1-170840) describes a system in which the non-nebulized component of a spray is led into a U-shape drain where hydrides are generated by the addition of a reducing agent. The aerosol part and the hydrides are delivered to the atomic spectrometer for determination. However, memory effects and sample volume control are difficult to maintain in this device.
A disclosure by Borgnon and Cadet, in a paper entitled, “Analyse des elements Hg, Se, As, Sn, Sb, et Bi en vapeur froide et hydrures par spectrométrie d'emission” Analusis, Vol. 16, pp. 77-80 (1988), is reported in U.S. Pat. No. 5,939,648 to represent a system that delivers hydrides and nebulized components. The Analusis paper, however, presents no claims for the determination of elements other than those delivered by vapor generation, since the nebulization of samples containing other elements results in excessive noise, and the device is described as having significant memory effects for several elements.
Similar work was disclosed by Li et al. in a paper entitled “Simultaneous determination of hydride and non-hydride forming elements by inductively coupled plasma atomic emission spectrometry,” published in Analytical Proceedings, Vol. 29 pp. 438-439 (1992). The Li paper discloses a device in which hydrides are generated by mixing a solution of acid with the sample solution and then with a solution of sodium borohydride in a manifold (called a “chemifold” in this publication). The generated hydrides are swept into the spray chamber where a second part of the sample is introduced by nebulization. The two components are then delivered to the atomic spectrometry instrumentation. No further work was reported by the authors, who indicated that, “Certainly more experimental data are required before routine environmental analyses can be carried out with confidence with this method.”
The foregoing inventions anticipate an invention, described in U.S. Pat. No. 5,939,648, assigned to Instruments S.A., Paris, France. In this device, hydride generation takes place within a spray chamber where the sample is introduced by nebulization. The sample portion that is not nebulized is collected in a modified drain where acid and sodium borohydride are introduced to generate the hydrides. In addition, the hydrogen, generated by the decomposition of the borohydride, is used to carry the hydrides into the gas phase, from where they are transported by a vector gas to the atomic spectrometer. For this device, the efficiency of transfer of the species to the gas phase would be reduced without the generation of hydrogen as an integral part of the operation.
Other inventors have used finely-divided gas bubbles that are generated by passage through a frit to separate the volatile species from the solution. See Brindle, Ian D. and Shaoguang Zheng, “A comparison of gas-liquid separators for the determination of mercury by cold-vapor sequential injection atomic absorption spectrometry”, Spectrochimica Acta Part B, Vol. 51 at pp. 1777-1780 (1996). A problem with the frit type of device is that the noisiness of the signal increases as the concentration of the species to be measured increases. Vapor generation with simultaneous nebulization of solution has been previously used to enhance the signals generated in the plasma when a solution of easily ionized element, such as potassium, is nebulized simultaneously with the generation of vapor (Brindle, Ian D. and Xiao-chun Le, “Application of Signal Enhancement by Easily Ionized Elements in Hydride Generation Direct Current Plasma Atomic Emission Spectrometric Determination of Arsenic, Antimony, Germanium, Tin, and Lead,” Analytical Chemistry, Vol. 61, pp. 1175-1178 (1989). A paper by Moor, et al. (Journal of Analytical Atomic Spectrometry, Vol. 15(2), pp. 143-49 (2000)) describes a system in which reagent and sample are mixed immediately prior to their being introduced into a spray chamber.
More conventional gas-liquid separators that use frits or other means to separate vapors from solutions for atomic spectrometry were not designed to operate simultaneously with a conventional nebulizer.